Change brightness/contrast for part of image

I am able to change the brightness/contrast for part of an image by using, for example, the Rectangle Select tool. I want to be a little trickier though. I would like to use the rectangle select tool, but have the effect of the brightness/contrast apply 100% at the bottom of the rectangle and say 10% at the top, with gradations from 100 to 10 from bottom to top. Is this possible?